get any na/nb, worry about other stuff later. miata is cheap, parts are cheap, upgrades are cheap.
Well, if you intend to use it on track day, you can look for NB with bigger brakes up front. Those brakes aren't bad at all, so lot of time will pass until you'll start pushing it hard enough to need any brake upgrade from miata sports brakes. That being said, even stock brakes will be enough for starters, so don't worry if you won't find any sports brakes offer. I'm taking my stock miata with small brakes on the track and it stays alive, my main problem now is bit of fade from shitty brake pads.
If you'll find 1.8 NB with torsen (i think every 1.8 is with torsen, isn't it?) and sports packages (suspension, brakes), it will be amazing car ready for any kind of track abuse you want. If you won't find this kind of offer, any other kind of miata will be alright as well.